In theory I would like to agree with this, but Firefox and Chrome say otherwise. In the last year, they've gone from speedy on a Core2Duo to bloated and slow on a modern i5.
You want to know the only machine I have that Firefox runs well on including my extensions? A 5Ghz (heavily overclocked, of course) i5... Even the i5 in my laptop (which turbo boosts up to 3GHz) struggles at times.
Yes, I know, I should pare down my selection of extensions etc. - but shouldn't a 3GHz i5 with an SSD and 16 gigs of RAM be able to run a simple browser without issues? And more often than not, it's the websites themselves that are slowing everything down - Slashdot is fine, but loading multimedia-heavy pages actually puts a noticeable load on a modern laptop CPU, which is quite different from a few years ago.